Fuel, such as light oil, is used in a compression ignition type internal combustion engine, such as a diesel engine. However, the Cetane numbers of commercially-supplied fuels are not necessarily constant. Thus, usage of fuel comprising a Cetane number other than a predetermined Cetane number may bring about a misfire and white smoke, as well as an increase in harmful substances contained in exhaust gas, and reduction of the fuel consumption performance. In an approach to solving such a problem, a Cetane number may be estimated for the fuel actually used in an internal combustion engine, and the estimation result may be reflected in the operation control of the internal combustion engine.
There are various methods proposed to estimate a Cetane number of fuel, such as those disclosed in Patent Documents 1 and 2. In Patent Document 1, if an internal combustion engine is in an idle state, a fuel-injection timing appears to be gradually retarded to give rise to a combustion state gradually that is unstable and to deliberately bring about a misfire. In response, the amount of retard at which the misfire occurs is determined, and the Cetane number of the fuel is estimated on the basis of a map that specifies in advance a relationship between the amount of retard and the Cetane number.
In Patent Document 2, a Cetane number appears to be estimated on the basis of a fuel-injection timing at which a relationship “ΔP/ΔCA=a” is substantially satisfied. ΔP may correspond to a difference between: an in-cylinder pressure peak value after a top dead center of compression due to combustion brought about by fuel injection; and an in-cylinder pressure peak value of a top dead center of compression or in the vicinity of the same due, at least in part, to combustion responsive to pilot injection before fuel injection or compression of in-cylinder gas due to piston motion. ΔCA is a difference in crank angles corresponding to the respective in-cylinder pressure values.    Patent Document 1: JP2007-154699A    Patent Document 2: JP2009-144634A
In Patent Document 1, a Cetane number appears to be estimated on the basis of a misfire responsive to in a combustion state by retarding a fuel-injection timing. However, a misfire is a phenomenon that occurs responsive to the combustion state becoming unstable. Further, occurrence of a misfire in a combustion state may lead to a considerable decrease in the fuel consumption performance.
In Patent Document 2, the Cetane number is estimated on the basis of in-cylinder pressure peak values. Thus, it may be necessary to provide an additional in-cylinder pressure sensor in an internal combustion engine to implement the estimation method, which may result in an increase in the implementation cost.
Particular embodiments of claimed subject matter may provide a method and an apparatus for estimating a fuel-Cetane number that can be implemented with a simplified and inexpensive configuration that does not give rise to an unstable combustion state.